Thank you Howard for you response! However, I have a follow-up question:
Why are UnsafeMutableBufferPointer methods which modify not the buffer pointer
itself, but only the pointed-to memory, mutating at all?
UnsafeMutable(Buffer)Pointer already have non-mutating subscript setters, which
means that I can modify the pointed-to memory even if the buffer pointer itself
is a constant. For example, this compiles and runs without problems:
func foo(bufptr: UnsafeMutableBufferPointer<Int>) {
let tmp = bufptr[0]
bufptr[0] = bufptr[1]
bufptr[1] = tmp
}
var a = [1, 2, 3, 4, 5]
a.withUnsafeMutableBufferPointer { foo(bufptr: $0) }
Doing the same with swapAt() does not compile:
func bar(bufptr: UnsafeMutableBufferPointer<Int>) {
bufptr.swapAt(0, 1)
// error: cannot use mutating member on immutable value: 'bufptr' is a
'let' constant
}
which means that I have to use a variable copy:
func bar(bufptr: UnsafeMutableBufferPointer<Int>) {
var bufptr = bufptr
bufptr.swapAt(0, 1)
}
So my "feeling" is that methods (like swapAt) which modify the pointed-to
memory of an UnsafeMutableBufferPointer should be non-mutating.
That would then also allow (coming back to my original question) that
withUnsafeMutableBufferPointer() passes a _constant_ buffer pointer to the
closure, and _might_ make the check at
https://github.com/apple/swift/blob/master/stdlib/public/core/Arrays.swift.gyb#L1750
obsolete (which verifies that the closure did not modify the pointer or length).
I am probably overlooking something, so please let me know where I am wrong!
Regards, Martin
> On 9. Oct 2017, at 01:15, Howard Lovatt <[email protected]> wrote:
>
> If it isn't an `inout` then it is a `let` not a `var` and hence you can't
> call mutating methods on it. There is no 'invar' in Swift, best you can do is
> `inout`.
>
> -- Howard.
>
> On 9 October 2017 at 06:14, Martin R via swift-users <[email protected]>
> wrote:
> I wonder why the closure in the Array method
>
> mutating func withUnsafeMutableBufferPointer<R>(_ body: (inout
> UnsafeMutableBufferPointer<Array.Element>) throws -> R) rethrows -> R
>
> takes an _inout_ parameter. The closure is called with a pointer to the
> contiguous array storage, and I fail to imagine an example where it makes
> sense to assign a new value to the parameter.
>
> Any insights are welcome!
>
> Regards, Martin
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>
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